The 40% Rule: How Much Solar Your New Build Legally Needs
The 40% Rule: How Much Solar Your New Build Legally Needs
Checked against Approved Document L 2026 and current market data · September 2026
From 24 March 2027, every new home in England must carry on-site solar: panel area equal to at least 40% of the ground floor area, which lands around 3.4 kWp on a typical semi and 5.2 kWp on a detached. It’s a floor, not a target — undersized roofs, high-rises and heritage plots get documented adjustments, not a free pass. For buyers, the real question isn’t compliance, it’s whether the minimum array actually powers your house. Here’s the maths, the dates and the genuine exemptions.
For years, “how much solar does my new build need?” had an honest answer of none, legally — developers fitted token panels when a council policy demanded it, and skipped them when it didn’t. That era is over. England’s new Approved Document L, published on 24 March 2026, turns on-site solar into a functional requirement of the Building Regulations, and the number everyone in the trade now quotes is 40%.
I’ve spent the months since publication reading the document, the impact assessment and the compliance guidance, and talking to assessors who are already signing off designs against it. Here’s the plain-English version: what the rule says, how the sizing maths works on real house types, which dates actually matter, and — the bit buyers care about most — whether the compliance minimum is enough to live on.
What the 40% rule actually says
Three layers, in order of importance:
- A functional requirement. Every new dwelling in England must include on-site renewable electricity generation. This is regulation, not guidance — building control sign-off requires compliance modelling to prove it.
- A deemed-to-satisfy benchmark. Install solar PV with a panel area of at least 40% of the dwelling’s ground floor area and you satisfy that requirement without argument. Note it’s panel area, not roof area — a distinction that catches designers out weekly.
- A sizing equation. The required capacity is panel area multiplied by a reference efficiency of 0.22 kWp per square metre, which reflects where mainstream modules actually sit today (a 400–430 W panel occupies roughly 1.7–1.8 m²). The full wording lives in the Approved Document L, Volume 1: Dwellings.
The rule sits inside the wider Future Homes Standard package: new homes must emit at least 75% less carbon than the 2013 baseline, and fossil-fuel heating is effectively designed out, with heat pumps as the default. Crucially, you cannot buy your way out of the solar requirement with thicker insulation — the old trick of trading fabric upgrades against renewables is closed off in practice, which is why housebuilders are re-drawing roof layouts rather than arguing with assessors.
For context on how weak the old regime was: Part L 2021 only demanded a 31% carbon cut, and its notional spec let developers squeeze on one or two panels. NHBC registration data suggests around 42% of new homes already carried some PV under those outgoing rules — proof the box was being ticked, not that the panels were doing much. Compliance kits from that era typically land around 85% below the new 40% benchmark. If you bought a new build between 2022 and 2027, that gap is exactly why your “solar home” feels underwhelming — our guide to extending an undersized new-build array is the follow-on read.
The maths: from ground floor area to kWp
The calculation is short enough to do on a napkin. Take the heated floor area of the lowest storey. Multiply by 0.4. That’s your minimum panel area. Multiply by 0.22 kWp/m² and you have the minimum installed capacity.
| House type | Ground floor area | Min. panel area (40%) | Min. capacity | Typical output |
|---|---|---|---|---|
| 2-bed terrace | ~35–45 m² | 14–18 m² | ~3.1–4.0 kWp | ~3,300–4,200 kWh/yr |
| 3-bed semi | ~42.5 m² | 17 m² | ~3.4 kWp | ~3,570 kWh/yr |
| 4-bed detached | ~65 m² | 26 m² | ~5.2 kWp | ~5,460 kWh/yr |
| 5-bed executive | ~95 m² | 38 m² | ~7.6 kWp | ~7,980 kWh/yr |
Two details worth internalising. First, the requirement is per dwelling: in a block of flats the ground floor area is apportioned between homes on an attributable basis, rather than each flat claiming the whole footprint. Second, poor orientation doesn’t reduce the required kWp — an east/west split roof still owes the same capacity, it just generates less per kWp, which the compliance modelling picks up.
2026, 2027 or 2028? The dates untangled
Coverage of this rule quotes three different years, and all three are “correct” about different events. Here’s the sequence:
- 24 March 2026 — publication. Approved Documents L and F were laid before Parliament with a written statement, giving the industry its first fixed text.
- 24 March 2027 — commencement. From this date, every new home approved in England must comply. This is the date the rule “starts”.
- 24 September 2027 — higher-risk buildings. Building work connected with higher-risk buildings — broadly, blocks of 18 metres or more, or seven storeys or more — follows six months later than everyone else.
- 24 March 2028 — end of transition. Projects approved under the old rules get a 12-month grace window, but building work must have started by this date or the old approval lapses into the new regime.
So when a headline says solar becomes mandatory “from 2028”, it’s describing the tail of the transitional period, not the start. When a vendor page says 2026, it’s describing publication. And when your plot’s sales pack quotes a rule version, the question to ask is simple: which approval date does this site fall under? That single date decides whether your home gets a 3.4 kWp array or a two-panel token kit.
One more geography note: this is England’s regulation. Scotland, Wales and Northern Ireland are moving on broadly aligned timetables, with Wales reportedly running slightly ahead — but the 40% figure itself is an English deemed-to-satisfy benchmark, so check the devolved wording if your plot sits north or west of the border.
Exemptions: when the 40% can be relaxed
Requirement L3 — the legal clause behind all this — sets out three situations where a new dwelling doesn’t have to hit the 40% figure at all:
- Higher-risk buildings. Blocks of 18 metres or more, or seven or more storeys, containing two or more dwellings are excluded from Requirement L3 entirely; they’re dealt with under the separate higher-risk building regime instead.
- Genuine impossibility. Where no system capable of a reasonable output can be installed at all because of the building’s design or surroundings. This is a high bar — the official guidance is explicit that reduced generation from overshading, or from a north-facing roof, is not sufficient justification on its own, because the compliance calculation already prices the effect of shading and orientation into the required output.
- Equivalent off-site generation. If an equivalent amount of renewable electricity is made available to residents from a system that isn’t on the building or within its curtilage — a shared array elsewhere on the development, say — that satisfies the requirement without a single panel on the roof.
Separately, listed buildings, buildings in conservation areas and scheduled monuments don’t have to fully meet any of Part L’s energy-efficiency requirements — L3 included — if doing so would unacceptably alter the building’s character or appearance. Compliance is still required “to the extent reasonably practicable” instead. This is the route a mansard roof in a conservation area, or an infill plot beside a listed building, typically falls under.
Where none of those apply but a roof genuinely can’t take the full notional area — small, complex or heavily obstructed by chimneys, vents and the like — the fallback still isn’t zero. The developer installs PV covering the maximum “reasonably practicable roof area” at the reference efficiency, and records the technical justification in the SAP or Home Energy Model compliance file for building control. What you cannot do is claim a reduction on cost grounds, on the basis panels would spoil the elevation, or simply because a section of roof underperforms — Parliament priced the whole Future Homes Standard package, solar included, at roughly £4,350 per typical plot and decided that was acceptable.
A sales pack citing “reduced coverage due to overshading” or “north-facing aspect” as the reason for an undersized array is leaning on something that, on its own, isn’t a valid exemption. The calculation already accounts for reduced output from shading and orientation when it sets the target — it doesn’t let a developer shrink the panel area because of it. If a plot’s solar looks light, ask which of the three genuine exemptions above, or the reasonably-practicable-roof-area route, the design is actually relying on, and see the paperwork.
What to ask your developer before you reserve
If you’re buying a plot or an off-plan home between now and 2028, these eight questions separate a genuine FHS-ready build from a compliance exercise:
- What kWp is installed, and how does it compare to the 40% minimum? At-minimum is legal; above-minimum is a selling point.
- Which approval regime does this site fall under? Pre- or post-24 March 2027 decides everything.
- Will I get the MCS certificate at completion? No certificate, no SEG export income later, and no proof of compliant installation.
- Is there inverter headroom to extend? An inverter sized exactly to the minimum array makes future extensions costlier. Our extension guide shows why headroom is money.
- Is the house battery-ready? Spare consumer-unit ways, wiring routes and a designated location cost pennies during construction and thousands afterwards.
- Is it export-ready? Smart meter specification and an export MPAN path mean you can join the SEG on day one instead of month six.
- Who owns the panels? Outright, always. Any lease or “free solar” arrangement on a new build should end the conversation — see our piece on rent-a-roof contracts and house sales.
- What monitoring and warranty come with it? A 20-year insurance-backed warranty and a usable app, not a brochure screenshot.
Our fuller walkthrough for purchasers, buying a house with solar panels, covers the conveyancing side of the same checklist.
Why the minimum isn’t the optimum
Here’s the part developers won’t put in the brochure: a compliance array is sized by regulation, not by your life. Take the 3.4 kWp minimum on a typical semi — around 3,570 kWh a year on a decent south-facing roof. Now picture the home it’s serving: heat pump, induction hob, maybe an EV charger. All-electric households like that commonly draw well beyond 6,000 kWh a year, and crucially they draw it at breakfast and after 17:00, when the array is producing little or nothing.
Self-consumption shares reflect typical UK figures: around 40% without storage rising to roughly 78% with a battery. Generation assumes a south-facing 3.4 kWp array.
Read the bars and the strategy writes itself. Without storage, most of your compliant solar leaves the house at export prices — anywhere from single figures on a truly open, no-tie tariff up to around 16.5p on the best supplier-tied deals — while you buy evening power at the capped rate of 26.32p for October to December 2026. Add a battery and the same array quietly becomes worth half as much again, because the units stay home. Batteries are not mandatory under the Future Homes Standard, which is precisely why asking for battery-ready wiring at reservation stage is the single cheapest upgrade available to you. Our guides to retrofitting batteries and earning from home storage cover both halves of that decision.
What compliance solar is actually worth you
Put current prices on the 3.4 kWp minimum and the generation picture is genuinely good, even at compliance sizing. Self-consumed units at the capped 26.32p rate are worth around £376 a year without a battery, with the exported remainder adding somewhere in the range of £100–£230 a year depending on the tariff you’re signed up to. Fit a battery and the self-consumed share rises to about £733, with export income falling accordingly — a meaningfully better return from exactly the same panels.
One number I won’t pretend to know is what the solar itself cost the developer. Asked directly in Parliament, the government confirmed it has never priced solar PV as a standalone line: the ~£4,350 impact-assessment figure covers the whole Part L 2026 package — PV, heat pump, fabric, MVHR, windows and doors — bundled together, and a developer’s bulk-procurement cost per plot sits well below what any of us would pay at retail. What you can compare it against is a like-for-like retrofit: our 2026 cost guide has current retail install prices for a system this size, and against those figures the minimum array typically pays for itself well inside its warranty period. Tariffs and the cap move too — our price cap tracker is where I check the import side each quarter. Either way, the era of arguing about whether new builds need solar is over. The argument now is about whether the solar you get is worth having — and that one, thankfully, you still win by asking questions.
If you’re weighing a compliant new build against retrofitting an older home instead, the comparison changes shape completely: no mandate, but also no free roof works bundled into the mortgage. Our 2026 cost guide and new build solar hub are the two pages I’d read side by side before committing either way.
FAQs
When does the 40% rule start?
Approved Document L 2026 takes effect in England on 24 March 2027, with a 12-month transitional period: homes approved under the old rules must start on site by 24 March 2028 or lose the grace period. That’s why you’ll see 2026, 2027 and 2028 in coverage — publication date, commencement date, and end of transitional period respectively.
Does the 40% rule apply to flats?
Yes, per dwelling. For blocks of flats the ground-floor area is apportioned between dwellings on an attributable basis. Higher-risk blocks — broadly, 18 metres or more, or seven or more storeys, with two or more dwellings — are excluded from the requirement entirely; everything else either hits the coverage figure or installs the maximum reasonably practicable roof area and records the justification in the compliance file.
Is the compliance minimum enough to run the house on?
Usually not on its own. A 3.4 kWp minimum generates around 3,500–3,600 kWh a year, which covers a good chunk of an all-electric home’s demand but leaves evening peaks to the grid. That’s why asking for battery-ready wiring at build stage is the cheapest upgrade you’ll ever make on a new build.
Rule details checked against Approved Document L Volume 1 (Dwellings), the Future Homes Standard impact assessment and current compliance guidance, September 2026. Sizing examples assume south-facing roofs at typical pitch; your plot’s SAP or home-energy-model output is the figure building control will sign off.
